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How to make custom types from strings using ExpressibleByStringLiteral

Swift version: 5.1

Paul Hudson    @twostraws   

Swift’s ExpressibleByStringLiteral protocol lets us create any type directly from a string – as long as Swift understands what type you mean, you can create whatever you want.

For example, if you regularly hard-code URLs and are tired of force unwrapping them when you know they are definitely correct, you can make URL conform to ExpressibleByStringLiteral so that URLs can be created directly from strings:

extension URL: ExpressibleByStringLiteral {
    public init(stringLiteral value: String) {
        self = URL(string: value)!
    }
}

With that extension in place you can now write code like this:

let url: URL = "https://www.hackingwithswift.com"
print(url.absoluteString)

Notice that I’ve clearly marked url as being of type URL so that Swift doesn’t think it’s a regular string.

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Available from iOS 8.0

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